Pacing mode switching and rate response limit in a ventricular pacemaker

ABSTRACT

An intracardiac ventricular pacemaker having a motion sensor, a pulse generator and a control circuit coupled to the pulse generator and the motion sensor is configured to identify a ventricular systolic event, detect a ventricular passive filling event signal from the motion signal, and determine a time interval from the ventricular systolic event to the ventricular passive filling event. The pacemaker establishes a minimum pacing interval based on the time interval.

TECHNICAL FIELD

The disclosure relates to a ventricular pacemaker and associated methodfor switching between an atrial tracking ventricular pacing mode and anon-atrial tracking ventricular pacing mode and limiting the ventricularpacing rate based on a motion sensor signal.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one transvenous medical lead for positioning electrodes inone heart chamber, atrial or ventricular, or a dual chamber pacemakercoupled to two intracardiac leads for positioning electrodes in both anatrial and a ventricular chamber. Multi-chamber pacemakers are alsoavailable that may be coupled to three leads, for example, forpositioning electrodes for pacing and sensing in one atrial chamber andboth the right and left ventricles.

Intracardiac pacemakers have recently been introduced that areimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular sensing and pacing by an intracardiac ventricularpacemaker may adequately address some patient conditions, otherconditions may require atrial and ventricular (dual chamber) sensing forproviding atrial-synchronized ventricular pacing in order to maintain aregular heart rhythm.

SUMMARY

In general, the disclosure is directed to a ventricular pacemaker andtechniques for controlling the ventricular pacing mode by switchingbetween an atrial tracking pacing mode and a non-atrial tracking pacingmode. The ventricular pacemaker may be an intracardiac pacemaker and maybe configured to detect atrial systolic events from a motion signalproduced by a motion sensor included in the intracardiac ventricularpacemaker. The ventricular pacemaker, operating according to thetechniques disclosed herein, automatically switches between an atrialtracking pacing mode and a non-atrial tracking pacing mode according tomode-switching criteria in order to provide an appropriate ventricularpacing rate that supports the patient's metabolic need.

In one example, the disclosure provides an intracardiac ventricularpacemaker including a pulse generator, a motion sensor, and a controlcircuit. The pulse generator is configured to generate and deliverpacing pulses to a ventricle of a patient's heart via electrodes coupledto the pacemaker. The motion sensor is configured to produce a motionsignal. The control circuit is coupled to the motion sensor and thepulse generator and is configured to identify a ventricular systolicevent, detect a ventricular passive filling event signal from the motionsignal, determine a time interval from the ventricular systolic event tothe ventricular passive filling event and establish a minimum pacinginterval based on the time interval. The control circuit is furtherconfigured to determine a sensor indicated pacing rate interval based onthe motion signal, compare the sensor indicated pacing rate interval tothe minimum pacing interval, and control the pulse generator to delivera ventricular pacing pulse at the minimum pacing interval in response tothe sensor indicated pacing rate interval being less than the minimumpacing interval.

In another example, the disclosure provides a method performed by anintracardiac ventricular pacemaker having a motion sensor configured toproduce a motion signal. The method includes identifying a ventricularsystolic event by a control circuit of the pacemaker, detecting aventricular passive filling event signal from the motion signal by thecontrol circuit, determining a time interval from the ventricularsystolic event to the ventricular passive filling event, establishing aminimum pacing interval based on the time interval, determining a sensorindicated pacing rate interval based on the motion signal, comparing thesensor indicated pacing rate interval to the minimum pacing interval,and controlling a pulse generator to deliver a ventricular pacing pulseat the minimum pacing interval in response to the sensor indicatedpacing rate interval being less than the minimum pacing interval.

In another example, the disclosure provides a non-transitory,computer-readable medium storing a set of instructions, which, whenexecuted by a control circuit of an intracardiac ventricular pacemakerhaving a motion sensor configured to produce a motion signal, cause thepacemaker to identify a ventricular systolic event, detect a ventricularpassive filling event signal from the motion signal, determine a timeinterval from the ventricular systolic event to the ventricular passivefilling event, establish a minimum pacing interval based on the timeinterval, determine a sensor indicated pacing rate interval based on themotion signal, compare the sensor indicated pacing rate interval to theminimum pacing interval, and deliver a ventricular pacing pulse at theminimum pacing interval in response to the sensor indicated pacing rateinterval being less than the minimum pacing interval.

The details of one or more aspects of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the techniques described in this disclosurewill be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem that may be used to sense cardiac electrical signals and motionsignals induced by cardiac motion and flowing blood and provide pacingtherapy to a patient's heart.

FIG. 2A is a conceptual diagram of the intracardiac ventricularpacemaker shown in FIG. 1.

FIG. 2B is a conceptual diagram of another example of the intracardiacventricular pacemaker shown in FIG. 1.

FIG. 3 is a schematic diagram of an example configuration of thepacemaker of FIG. 2A.

FIG. 4 is an example of a motion sensor signal that may be acquired overa cardiac cycle by a motion sensor included in the ventricularintracardiac pacemaker of FIG. 1.

FIG. 5 is a flow chart of one method performed by an intracardiacventricular pacemaker for detecting the A4 event and controllingventricular pacing during an atrial tracking pacing mode.

FIG. 6 is an example of motion sensor signals acquired over twodifferent cardiac cycles.

FIG. 7 is an averaged motion sensor signal that may be determined byaveraging the motion sensor signal obtained over multiple cardiaccycles.

FIG. 8 is a flow chart of a method performed by an intracardiacventricular pacemaker for detecting atrial events and controllingatrial-synchronized ventricular pacing during an atrial tracking pacingmode according to one example.

FIG. 9 is a flow chart of a method for controlling the ventricularpacing mode by an intracardiac ventricular pacemaker according to oneexample.

FIG. 10 is a flow chart of a method performed by an intracardiacventricular pacemaker for establishing a minimum ventricular lower rateinterval during a non-tracking atrial pacing mode according to oneexample.

FIG. 11 is a flow chart of one method performed by an intracardiacventricular pacemaker for controlling ventricular pacing according toone example.

DETAILED DESCRIPTION

During atrial-synchronized ventricular pacing, ventricular pacing pulsesare delivered at an atrioventricular (AV) pacing interval following anatrial event to provide proper synchrony between the atrial contractionand the ventricular contraction. In order for a ventricular intracardiacpacemaker to provide atrial-synchronized ventricular pacing, theventricular intracardiac pacemaker needs to receive a signal indicatingthe timing of the atrial event in order to start the AV pacing interval.As the atrial rate increases or decreases, the ventricular pacing ratetracks the atrial rate. This atrial synchronized ventricular pacing isreferred to as an “atrial-tracking” pacing mode. In a non-atrialtracking ventricular pacing mode, the ventricular pacing pulses aredelivered at a ventricular lower rate (LR) pacing interval that isindependent of atrial events and does not track the atrial rate.Generally, an atrial tracking ventricular pacing mode is desirable whenthe sinus node of the heart is functioning normally in setting theintrinsic atrial rate or the atria are being paced at an appropriatepacing rate for the patient's level of physical activity.

Under some circumstances, however, it is undesirable to track the atrialrate, e.g., during atrial tachyarrhythmia such as atrial fibrillation oratrial flutter. As such, a method for switching between an atrialtracking ventricular pacing mode and a non-atrial tracking ventricularpacing mode is desirable. An intracardiac ventricular pacemaker isdisclosed herein that delivers ventricular pacing that tracks the atrialrate at appropriate times and does not track the atrial rate at timesthat atrial tracking is not appropriate. In this way, ventricular pacingis provided based on the patient's need and in a manner that does notresult in pacemaker mediated ventricular tachycardia.

An intracardiac ventricular pacemaker as disclosed herein may have amotion sensor for detecting atrial systolic mechanical events. Duringthe atrial tracking pacing mode, the AV pacing interval is started inresponse to detecting the atrial systolic mechanical event. Atrialsystolic mechanical event detection during one or both the atrialtracking pacing mode and the non-atrial tracking pacing mode may be usedto determine if pacing mode switching criteria are met for controllingwhen the pacemaker switches between the atrial tracking pacing mode andthe non-atrial tracking pacing mode. During the non-atrial trackingpacing mode, the ventricular intracardiac pacemaker may determine anactivity level of the patient from the motion sensor signal to providerate responsive ventricular pacing at a temporary LR pacing intervalthat is adjusted to meet the demand of the patient's physical activity.Techniques are disclosed herein for controlling switching between anatrial tracking ventricular pacing mode and a non-atrial trackingventricular pacing mode based on the motion sensor signal in anventricular intracardiac pacemaker. As described below, the ventricularintracardiac pacemaker limits the maximum ventricular pacing rate duringthe non-atrial tracking ventricular pacing mode, e.g., a VVIR pacingmode, based on the timing of cardiac events detected from the motionsensor signal.

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem 10 that may be used to sense cardiac electrical signals andmotion signals induced by patient physical activity, cardiac motion andflowing blood and provide pacing therapy to a patient's heart 8. IMDsystem 10 includes a right ventricular (RV) intracardiac pacemaker 14and may optionally include a right atrial (RA) intracardiac pacemaker 12in some examples. Pacemakers 12 and 14 may be transcatheter intracardiacpacemakers which may be adapted for implantation wholly within a heartchamber, e.g., wholly within the RV, wholly within the left ventricle(LV), wholly within the RA or wholly within the left atrium (LA) ofheart 8.

In the example of FIG. 1, pacemaker 12 is positioned along anendocardial wall of the RA, e.g., along the RA lateral wall or RAseptum. Pacemaker 14 is positioned along an endocardial wall of the RV,e.g., near the RV apex though other locations are possible. Thetechniques disclosed herein are not limited to the pacemaker locationsshown in the example of FIG. 1 and other positions and relativelocations in the heart 8 and from each other are possible. For example,a ventricular intracardiac pacemaker 14 may be positioned in the LV andconfigured to detect cardiac motion signals and deliveratrial-synchronized ventricular pacing to the LV using the techniquesdisclosed herein. Pacemaker 14 may be positioned within the rightventricle or left ventricle to provide respective right ventricular orleft ventricular pacing and for sensing motion signals by a motionsensor within the ventricular chamber. Pacemaker 12 may be positionedoutside or within the right atrium or left atrium to provide respectiveright atrial or left atrial pacing. Pacemakers 12 and 14 may be reducedin size compared to subcutaneously implanted pacemakers and may begenerally cylindrical in shape to enable transvenous implantation via adelivery catheter.

Pacemakers 12 and 14 are each capable of producing electricalstimulation pulses, e.g., pacing pulses, delivered to heart 8 via one ormore electrodes on the outer housing of the pacemaker. RA pacemaker 12is configured to sense a cardiac electrical signal from within the RAthat may be used to produce an RA intracardiac electrogram (EGM) signal.RV pacemaker 14 is configured to deliver RV pacing pulses and sense anRV cardiac electrical signal using housing based electrodes forproducing an RV EGM signal. The cardiac electrical signals may be sensedby the respective pacemaker 12 or 14 using the housing based electrodesthat are also used to deliver pacing pulses to the respective RA or RV.

In some examples, a patient may only require RV pacemaker 14 fordelivering ventricular pacing. In other examples, depending onindividual patient need, RA pacemaker 12 may be required for deliveringatrial pacing. The RV pacemaker 14 is configured to control the deliveryof ventricular pacing pulses to the RV in a manner that promotessynchrony between the RA contraction and the RV contraction, e.g., bymaintaining a target AV pacing interval between atrial events, which maybe sensed from the motion signal, and ventricular pacing pulses. Thatis, the RV pacemaker 14 controls RV pacing pulse delivery to maintain adesired AV interval between an atrial activation corresponding to atrialsystole (intrinsic or pacing-evoked) and the subsequent ventricularpacing pulse delivered to cause ventricular depolarization.

According to the techniques described herein, atrial activations aredetected by RV pacemaker 14 from a motion sensor signal that includesmotion signals caused by ventricular and atrial events. For example,acceleration of blood flowing into the RV through the tricuspid valve 16between the RA and RV caused by atrial contraction, sometimes referredto as the “atrial kick,” is detected by RV pacemaker 14 from the signalproduced by a motion sensor, for example an accelerometer, included inRV pacemaker 14. Other cardiac motion signals that may be detected by RVpacemaker 14 are described below in conjunction with FIG. 4 and mayinclude motion caused by ventricular contraction, motion caused byventricular relaxation, and motion caused by passive filling of theventricle.

Atrial P-waves that are attendant to atrial depolarization arerelatively low amplitude, far-field signals in the RV cardiac electricalsignal received by pacemaker 14 compared to the amplitude of near-fieldR-waves. The far-field P-wave can be difficult to reliably detect fromthe cardiac electrical signal acquired by RV pacemaker 14. As such,atrial-synchronized ventricular pacing by RV pacemaker 14 may not bereliable when atrial events are sensed solely from a cardiac electricalsignal received by RV pacemaker 14. According to the techniquesdisclosed herein, the RV pacemaker 14 includes a motion sensor, such asan accelerometer, and is configured to detect an atrial systolic eventcorresponding to the mechanical activation (contraction) of the atriapresent in the motion signal produced by the motion sensor. Ventricularpacing pulses are synchronized to the atrial event that is detected fromthe motion signal by setting a programmable AV pacing interval inresponse to detecting the atrial systolic event. The AV pacing intervalcontrols the timing of the ventricular pacing pulse relative to thedetected atrial systolic event. Detection of the atrial systolic eventused to synchronize ventricular pacing pulses to atrial systolic eventsmay include detection of other cardiac event motion signals, e.g.,ventricular events, in order to positively identify the atrial systolicevent.

A target AV interval may be a programmed value selected by a clinicianand is the time interval from the detection of the atrial event from themotion sensor signal until delivery of the ventricular pacing pulse. Insome instances, the target AV interval may be started from the time theatrial systolic event is detected from a motion sensor signal orstarting from an identified fiducial point of the atrial systolic eventsignal in the motion sensor signal. The target AV interval may beidentified as being hemodynamically optimal for a given patient based onclinical testing or assessments of the patient or based on clinical datafrom a population of patients.

Pacemakers 12 and 14 may each be capable of bidirectional wirelesscommunication with an external device 20 for programming the AV pacinginterval and other pacing control parameters as well as mechanical eventsensing parameters utilized for detecting ventricular mechanical eventsand the atrial systolic event from the motion sensor signal. Aspects ofexternal device 20 may generally correspond to the externalprogramming/monitoring unit disclosed in U.S. Pat. No. 5,507,782(Kieval, et al.), hereby incorporated herein by reference in itsentirety. External device 20 is often referred to as a “programmer”because it is typically used by a physician, technician, nurse,clinician or other qualified user for programming operating parametersin pacemakers 12 and 14. External device 20 may be located in a clinic,hospital or other medical facility. External device 20 may alternativelybe embodied as a home monitor or a handheld device that may be used in amedical facility, in the patient's home, or another location. Operatingparameters, including sensing and therapy delivery control parametersand pacing mode switching criteria, may be programmed into pacemaker 14using external device 20.

External device 20 is configured for bidirectional communication withimplantable telemetry circuitry included in RV pacemaker 14 and RApacemaker 12 (when present). External device 20 may establish a wirelessradio frequency (RF) communication link 22 with RA pacemaker 12 andwireless RF communication link 24 with RV pacemaker 14 using acommunication protocol that appropriately addresses the targetedpacemaker 12 or 14. Communication links 22 and 24 may be establishedusing an RF link such as BLUETOOTH®, Wi-Fi, Medical ImplantCommunication Service (MICS) or other communication bandwidth. In someexamples, external device 20 may include a programming head that isplaced proximate pacemaker 12 or 14 to establish and maintain acommunication link, and in other examples external device 20 andpacemakers 12 and 14 may be configured to communicate using a distancetelemetry algorithm and circuitry that does not require the use of aprogramming head and does not require user intervention to maintain acommunication link. An example RF telemetry communication system thatmay be implemented in system 10 is generally disclosed in U.S. Pat. No.5,683,432 (Goedeke, et al.), hereby incorporated herein by reference inits entirety.

External device 20 may display data and information relating topacemaker functions to a user for reviewing pacemaker operation andprogrammed parameters as well as EGM signals transmitted from pacemaker14 or pacemaker 12, motion sensor signals acquired by pacemaker 14, orother physiological data that is acquired by and retrieved frompacemakers 12 and/or 14 during an interrogation session.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communications network via a telemetry circuit thatincludes a transceiver and antenna or via a hardwired communication linefor transferring data to a remote database or computer to allow remotemanagement of the patient. Remote patient management systems including aremote patient database may be configured to utilize the presentlydisclosed techniques to enable a clinician to review EGM, motion sensor,and marker channel data and authorize programming of sensing and therapycontrol parameters in RV pacemaker 14, e.g., after viewing a visualrepresentation of EGM, motion sensor signal and marker channel data.

Pacemaker 12 and pacemaker 14 may or may not be configured tocommunicate directly with each other. When pacemakers 12 and 14 areconfigured to communicate with each other, communication may beminimized in order to conserve battery life of the intracardiacpacemakers 12 and 14. As such, communication may not occur on abeat-by-beat basis between the RA pacemaker 12 and RV pacemaker 14 forcommunicating when the other pacemaker is sensing cardiac events or whenit is delivering pacing pulses in a respective heart chamber. Asdisclosed herein, RV pacemaker 14, however, is configured to detectatrial events as often as beat-by-beat from a motion sensor signal,without requiring communication signals from RA pacemaker 12. Atrialevent detection for controlling atrial-synchronized ventricular pacingand for determining when to switch to a non-atrial tracking pacing mode,e.g., a VVIR pacing mode, is performed by RV pacemaker 14.

FIG. 2A is a conceptual diagram of the intracardiac RV pacemaker 14shown in FIG. 1. RV pacemaker 14 includes electrodes 162 and 164 spacedapart along the housing 150 of pacemaker 14 for sensing cardiacelectrical signals and delivering pacing pulses. Electrode 164 is shownas a tip electrode extending from a distal end 102 of pacemaker 14, andelectrode 162 is shown as a ring electrode along a mid-portion ofhousing 150, for example adjacent proximal end 104. Distal end 102 isreferred to as “distal” in that it is expected to be the leading end aspacemaker 14 is advanced through a delivery tool, such as a catheter,and placed against a targeted pacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. In alternative embodiments, pacemaker 14 mayinclude two or more ring electrodes, two tip electrodes, and/or othertypes of electrodes exposed along pacemaker housing 150 for deliveringelectrical stimulation to heart 8 and sensing cardiac electricalsignals. Electrodes 162 and 164 may be, without limitation, titanium,platinum, iridium or alloys thereof and may include a low polarizingcoating, such as titanium nitride, iridium oxide, ruthenium oxide,platinum black among others. Electrodes 162 and 164 may be positioned atlocations along pacemaker 14 other than the locations shown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated.Electrode 164 may serve as a cathode electrode and be coupled tointernal circuitry, e.g., a pacing pulse generator and cardiacelectrical signal sensing circuitry, enclosed by housing 150 via anelectrical feedthrough crossing housing 150. Electrode 162 may be formedas a conductive portion of housing 150 as a ring electrode that iselectrically isolated from the other portions of the housing 150 asgenerally shown in FIG. 2A. In other examples, the entire periphery ofthe housing 150 may function as an electrode that is electricallyisolated from tip electrode 164, instead of providing a localized ringelectrode such as anode electrode 162. Electrode 162 formed along anelectrically conductive portion of housing 150 serves as a return anodeduring pacing and sensing.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker14 as described below in conjunction with FIG. 3. A motion sensor may beimplemented as an accelerometer enclosed within housing 150 in someexamples. The accelerometer provides a signal to a processor included incontrol electronics subassembly 152 for signal processing and analysisfor detecting atrial systolic events for timing ventricular pacingpulses as described below.

Housing 150 further includes a battery subassembly 160, which providespower to the control electronics subassembly 152. Battery subassembly160 may include features of the batteries disclosed in commonly-assignedU.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S. Pat. No. 8,541,131(Lund, et al.), both of which are hereby incorporated by referenceherein in their entirety.

Pacemaker 14 may include a set of fixation tines 166 to secure pacemaker14 to patient tissue, e.g., by actively engaging with the ventricularendocardium and/or interacting with the ventricular trabeculae. Fixationtines 166 are configured to anchor pacemaker 14 to position electrode164 in operative proximity to a targeted tissue for deliveringtherapeutic electrical stimulation pulses. Numerous types of activeand/or passive fixation members may be employed for anchoring orstabilizing pacemaker 14 in an implant position. Pacemaker 14 mayinclude a set of fixation tines as disclosed in commonly-assigned U.S.Pat. No. 9,775,982 (Grubac, et al.), hereby incorporated herein byreference in its entirety.

Pacemaker 14 may optionally include a delivery tool interface 158.Delivery tool interface 158 may be located at the proximal end 104 ofpacemaker 14 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 14 at an implant location withina ventricular heart chamber during an implantation procedure.

FIG. 2B is a conceptual diagram of another example of RV pacemaker 14.In FIG. 2B, RV pacemaker 14 includes a proximal sensing extension 165extending away from housing 150 and carrying a pair of sensingelectrodes 167 and 168. The proximal sensing extension 165 may becoupled to the housing 150 for positioning a return sensing electrode167 or 168 which may be paired with distal electrode 164 at an increasedinter-electrode distance compared to the inter-electrode spacing ofhousing-based electrodes 162 and 164. The increased inter-electrodedistance may facilitate sensing of far-field atrial signals such asP-waves attendant to atrial depolarization.

Alternatively, electrodes 167 and 168 may form a sensing electrode pairfor sensing atrial P-waves. When distal end 102 is fixed along the RVapex, sensing extension 165 may extend toward the RA thereby positioningelectrodes 167 and 168 nearer the atrial tissue for sensing far-fieldatrial P-waves. One electrode 167 may be coupled to sensing circuitryenclosed in housing 150 via an electrical feedthrough crossing housing150, and one electrode 168 may be coupled to housing 150 to serve as aground electrode.

FIG. 3 is a schematic diagram of an example configuration of pacemaker14 shown in FIG. 1. Pacemaker 14 includes a pulse generator 202, asensing circuit 204, a control circuit 206, memory 210, telemetrycircuit 208, motion sensor 212 and a power source 214. Motion sensor 212is implemented as an accelerometer in the examples described herein andmay also be referred to herein as “accelerometer 212.” Motion sensor 212is not limited to being an accelerometer, however, and other motionsensors may be utilized successfully in pacemaker 14 for detectingcardiac motion signals according to the techniques described herein.Examples of motion sensors that may be implemented in pacemaker 14include piezoelectric sensors and micro electro-mechanical systems(MEMS) devices.

Motion sensor 212 may be a multi-axis sensor, e.g., a two-dimensional orthree-dimensional sensor, with each axis providing a signal that may beanalyzed individually or in combination for detecting cardiac mechanicalevents. Motion sensor 212 produces an electrical signal correlated tomotion or vibration of sensor 212 (and pacemaker 14), e.g., whensubjected to flowing blood and cardiac motion. Motion sensor 212 may bea one-dimensional, single axis accelerometer, two-dimensional orthree-dimensional multi-axis accelerometer. One example of anaccelerometer for use in implantable medical devices is generallydisclosed in U.S. Pat. No. 5,885,471 (Ruben, et al.), incorporatedherein by reference in its entirety. An implantable medical devicearrangement including a piezoelectric accelerometer for detectingpatient motion is disclosed, for example, in U.S. Pat. No. 4,485,813(Anderson, et al.) and in U.S. Pat. No. 5,052,388 (Sivula, et al.), bothof which patents are hereby incorporated by reference herein in theirentirety. Examples of three-dimensional accelerometers that may beimplemented in pacemaker 14 and used for detecting cardiac mechanicalevents from a motion signal using the presently disclosed techniques aregenerally described in U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat.No. 6,044,297 (Sheldon), both of which are incorporated herein byreference in their entirety. Other accelerometer designs may be used forproducing an electrical signal that is correlated to motion imparted onpacemaker 14 due to ventricular and atrial events.

In addition to being subjected to cardiac motion, the motion sensor 212is subjected to patient body motion during physical activity such aswalking, driving, exercise, changing body posture, household chores andother activities of daily living. The motion sensor signal may be usedby control circuit 206 for detecting cardiac mechanical events asdescribed in greater detail below as well as determining the patient'sbody posture and/or a metric of patient physical activity. Techniquesfor monitoring patient body posture and patient physical activity froman accelerometer signal are generally disclosed in theabove-incorporated references. While examples presented herein refer toa single motion sensor, it is recognized that motion sensor 212 may beimplemented as two or more individual motion sensors with one motionsensor providing a signal from which a patient activity metric isdetermined and another motion sensor providing a signal from whichatrial events are detected.

The various circuits represented in FIG. 3 may be combined on one ormore integrated circuit boards which include a specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, state machine or other suitablecomponents that provide the described functionality.

Sensing circuit 204 is configured to receive a cardiac electrical signalvia electrodes 162 and 164 by a pre-filter and amplifier circuit 220.Pre-filter and amplifier circuit may include a high pass filter toremove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a widebandfilter having a passband of 2.5 Hz to 100 Hz to remove DC offset andhigh frequency noise. Pre-filter and amplifier circuit 220 may furtherinclude an amplifier to amplify the “raw” cardiac electrical signalpassed to analog-to-digital converter (ADC) 226. ADC 226 may pass amulti-bit, digital electrogram (EGM) signal to control circuit 206 foruse by atrial event detector circuit 240 in identifying ventricularelectrical events (e.g., R-waves or T-waves) and/or atrial electricalevents, e.g., P-waves. Identification of cardiac electrical events maybe used in algorithms for detecting atrial systolic events from themotion sensor signal. The digital signal from ADC 226 may be passed torectifier and amplifier circuit 222, which may include a rectifier,bandpass filter, and amplifier for passing a cardiac signal to R-wavedetector 224.

R-wave detector 224 may include a sense amplifier or other detectioncircuitry that compares the incoming rectified, cardiac electricalsignal to an R-wave detection threshold, which may be an auto-adjustingthreshold. When the incoming signal crosses the R-wave detectionthreshold, the R-wave detector 224 produces an R-wave sensed eventsignal (Rsense) that is passed to control circuit 206. In otherexamples, R-wave detector 224 may receive the digital output of ADC 226for detecting R-waves by a comparator, morphological signal analysis ofthe digital EGM signal or other R-wave detection techniques. R-wavesensed event signals passed from R-wave detector 224 to control circuit206 may be used for scheduling ventricular pacing pulses by pace timingcircuit 242 and for use in identifying the timing of ventricularelectrical events in algorithms performed by atrial event detectorcircuit 240 for detecting atrial systolic events from a signal receivedfrom motion sensor 212.

Control circuit 206 may include an atrial event detector circuit 240,pace timing circuit 242, and processor 244. Atrial event detectorcircuit 240 is configured to detect atrial mechanical events from asignal received from motion sensor 212. As described below, one or moreventricular mechanical events may be detected from the motion sensorsignal in a given cardiac cycle to facilitate positive detection of theatrial systolic event from the motion sensor signal during theventricular cycle.

Control circuit 206 may receive R-wave sensed event signals and/ordigital cardiac electrical signals from sensing circuit 204 for use indetecting and confirming cardiac events and controlling ventricularpacing. For example, R-wave sensed event signals may be passed to pacetiming circuit 242 for inhibiting scheduled ventricular pacing pulses orscheduling ventricular pacing pulses when pacemaker 14 is operating in anon-atrial tracking ventricular pacing mode. R-wave sensed event signalsmay also be passed to atrial event detector circuit 240 for use insetting ventricular event detection windows and/or atrial eventrefractory periods.

Atrial event detector circuit 240 receives a motion signal from motionsensor 212 and may start an atrial refractory period in response to aventricular electrical event, e.g., an R-wave sensed event signal fromsensing circuit 204 or delivery of a pacing pulse by pulse generator202. Atrial event detector circuit 240 may be configured to determine ifthe motion sensor signal satisfies atrial mechanical systolic eventdetection criteria outside of the refractory period. The motion sensorsignal during the refractory period may be monitored by atrial eventdetector circuit 240 for the purposes of detecting ventricularmechanical events, which may be used for confirming or validating atrialsystolic event detection and/or setting atrial systolic event detectioncontrol parameters. As such, ventricular mechanical event detectionwindows may be set during the atrial refractory period and may be setaccording to predetermined time intervals following identification of aventricular electrical event. The timing and detection of theventricular mechanical events may be used to update the atrialrefractory period and/or an atrial systolic detection thresholdamplitude and may be used to confirm detection of the atrial systolicevent occurring subsequent to ventricular mechanical events.

Atrial event detector circuit 240 may pass an atrial event detectionsignal to processor 244 and/or pace timing circuit 242. Pace timingcircuit 242 (or processor 244) may additionally receive R-wave sensedevent signals from R-wave detector 224 for use in controlling the timingof pacing pulses delivered by pulse generator 202. Processor 244 mayinclude one or more clocks for generating clock signals that are used bypace timing circuit 242 to time out an AV pacing interval that isstarted upon receipt of an atrial event detection signal from atrialevent detector circuit 240. Pace timing circuit 242 may include one ormore pacing escape interval timers or counters that are used to time outthe AV pacing interval, which may be a programmable interval stored inmemory 210 and retrieved by processor 244 for use in setting the AVpacing interval used by pace timing circuit 242. Techniques forcontrolling atrial-synchronized ventricular pacing using a motion sensorsignal are generally disclosed in U.S. Pat. No. 9,399,140, (Yong, etal.), incorporated herein by reference in its entirety.

Pace timing circuit 242 may additionally include a LR pacing intervaltimer for controlling a minimum ventricular pacing rate in the absenceof detected atrial events. For example, if an atrial systolic event isnot detected from the motion sensor signal triggering a ventricularpacing pulse at the programmed AV pacing interval, a ventricular pacingpulse may be delivered by pulse generator 202 upon expiration of the LRpacing interval to prevent ventricular asystole and maintain a minimumventricular rate.

During a non-atrial tracking pacing mode, the LR pacing interval timeris used to control the rate of ventricular pacing pulses that aredelivered independent of the atrial rate. The LR pacing interval may beset to a minimum or base LR pacing interval to maintain a minimumventricular rate and may be adjusted to a temporary LR pacing intervalto provide rate responsive pacing. A sensor indicated rate (SIR) (alsoreferred to herein as a sensor indicated pacing interval) may bedetermined based on the patient's physical activity level. In someinstances, the SIR may be determined from the motion sensor signalreceived from motion sensor 212. In other examples, an SIR may bedetermined by another sensor, such as a temperature sensor or the like.A temporary LR pacing interval set based on a sensor indicated rate(SIR) provides ventricular pacing pulses at a rate greater than theminimum or base pacing rate.

The higher ventricular rate support is provided according to thepatient's metabolic demand during periods of non-resting physicalactivity based on the SIR. The use of an accelerometer in anintracardiac pacemaker for obtaining a patient activity signal isgenerally disclosed in U.S. Pat. No. 9,814,887 (Nikolski, et al.),incorporated herein by reference in its entirety. The use of a patientactivity signal for determining a SIR and providing rate-responsivepacing is generally disclosed in U.S. Pat. No. 5,720,769 (van Oort) andU.S. Pat. No. 7,031,772 (Condie, et al.), both incorporated herein byreference in its entirety.

Processor 244 may retrieve other programmable pacing control parameters,such as pacing pulse amplitude and pacing pulse width that are passed topulse generator 202 for controlling pacing pulse delivery. In additionto providing control signals to pace timing circuit 242 and pulsegenerator 202 for controlling pacing pulse delivery, processor 244 mayprovide sensing control signals to sensing circuit 204, e.g., R-wavesensing threshold, sensitivity, various blanking and refractoryintervals applied to the cardiac electrical signal, and atrial eventdetection control signals to atrial event detector circuit 240 for usein detecting and confirming atrial systolic events, e.g., ventricularevent detection windows, atrial refractory period, detection thresholdamplitudes applied to the motion sensor signal, and any other atrialevent detection criteria applied by circuitry included in atrial eventdetector circuit 240.

The functions attributed to pacemaker 14 herein may be embodied as oneor more processors, controllers, hardware, firmware, software, or anycombination thereof. Depiction of different features as specificcircuitry is intended to highlight different functional aspects and doesnot necessarily imply that such functions must be realized by separatehardware, firmware or software components or by any particular circuitarchitecture. Rather, functionality associated with one or more circuitsdescribed herein may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, atrial systolic event detection fromthe motion sensor signal and ventricular pacing control operationsperformed by pacemaker 14 may be implemented in control circuit 206executing instructions stored in memory 210 and relying on input fromsensing circuit 204 and motion sensor 212.

The operation of circuitry included in pacemaker 14 as disclosed hereinshould not be construed as reflective of a specific form of hardware,firmware and software necessary to practice the techniques described. Itis believed that the particular form of software, hardware and/orfirmware will be determined primarily by the particular systemarchitecture employed in the pacemaker 14 and by the particular sensingand therapy delivery circuitry employed by the pacemaker 14. Providingsoftware, hardware, and/or firmware to accomplish the describedfunctionality in the context of any modern pacemaker, given thedisclosure herein, is within the abilities of one of skill in the art.

Pulse generator 202 generates electrical pacing pulses that aredelivered to the RV of the patient's heart via cathode electrode 164 andreturn anode electrode 162. Pulse generator 202 may include chargingcircuit 230, switching circuit 232 and an output circuit 234. Chargingcircuit 230 may include a holding capacitor that may be charged to apacing pulse amplitude by a multiple of the battery voltage signal ofpower source 214 under the control of a voltage regulator. The pacingpulse amplitude may be set based on a control signal from controlcircuit 206. Switching circuit 232 may control when the holdingcapacitor of charging circuit 230 is coupled to the output circuit 234for delivering the pacing pulse. For example, switching circuit 232 mayinclude a switch that is activated by a timing signal received from pacetiming circuit 242 upon expiration of an AV pacing interval (or LRpacing interval during a non-atrial tracking pacing mode) and keptclosed for a programmed pacing pulse duration to enable discharging ofthe holding capacitor of charging circuit 230. The holding capacitor,previously charged to the pacing pulse voltage amplitude, is dischargedacross electrodes 162 and 164 through the output capacitor of outputcircuit 234 for the programmed pacing pulse duration. Examples of pacingcircuitry generally disclosed in U.S. Pat. No. 5,507,782 (Kieval, etal.) and in U.S. Pat. No. 8,532,785 (Crutchfield, et al.), both of whichpatents are incorporated herein by reference in their entirety, may beimplemented in pacemaker 14 for charging a pacing capacitor to apredetermined pacing pulse amplitude under the control of controlcircuit 206 and delivering a pacing pulse.

Memory 210 may include computer-readable instructions that, whenexecuted by control circuit 206, cause control circuit 206 to performvarious functions attributed throughout this disclosure to pacemaker 14.The computer-readable instructions may be encoded within memory 210.Memory 210 may include any non-transitory, computer-readable storagemedia including any volatile, non-volatile, magnetic, optical, orelectrical media, such as a random access memory (RAM), read-only memory(ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or other digital media with the sole exceptionbeing a transitory propagating signal. Memory 210 may store timingintervals and other data used by control circuit 206 to control theventricular pacing mode and delivery of pacing pulses by pulse generator202.

Power source 214 provides power to each of the other circuits andcomponents of pacemaker 14 as required. Control circuit 206 may executepower control operations to control when various circuits or componentsare powered to perform various pacemaker functions. Power source 214 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 214 and other pacemaker circuits and components are notshown in FIG. 3 for the sake of clarity. However, power source 214 is tobe understood to provide power to pulse generator 202, sensing circuit204, telemetry circuit 208, memory 210, motion sensor 212 and controlcircuit 206 as needed. For example, power source 214 may provide powerto the charging circuit 230 for charging one or more holding capacitorsand to switching circuit 232 for controlling the discharge of theholding capacitors through output circuit 234.

Telemetry circuit 208 includes a transceiver 209 and antenna 211 fortransferring and receiving data, e.g., via a radio frequency (RF)communication link. Telemetry circuit 208 may be capable ofbi-directional communication with external device 20 (FIG. 1) asdescribed above. Motion sensor signals and cardiac electrical signals,and/or data derived therefrom may be transmitted by telemetry circuit208 to external device 20. Programmable control parameters andalgorithms for performing atrial and ventricular event detection fromthe motion sensor signal and for ventricular pacing control may bereceived by telemetry circuit 208 and stored in memory 210 for access bycontrol circuit 206.

FIG. 4 is an example of a motion sensor signal 250 that may be acquiredby motion sensor 212 over a cardiac cycle. Vertical dashed lines 252 and262 denote the timing of two consecutive ventricular events (intrinsicventricular depolarizations and/or ventricular pacing pulses), markingthe respective beginning and end of the ventricular cycle 251. Themotion signal includes an A1 event 254, an A2 event 256, an A3 event 258and an A4 event 260. The A1 event 254 is an acceleration signal (in thisexample when motion sensor 250 is implemented as an accelerometer) thatoccurs during ventricular contraction and marks the approximate onset ofventricular mechanical systole. The A1 event is also referred to hereinas a “ventricular contraction event.” The A2 event 256 is anacceleration signal that occurs during ventricular relaxation and marksthe approximate offset or end of ventricular mechanical systole. The A2event is also referred to herein as the “ventricular relaxation event.”The A3 event 258 is an acceleration signal that occurs during passiveventricular filling and marks ventricular mechanical diastole. The A3event is also referred to herein as the “ventricular passive fillingevent.” Since the A2 event occurs with the end of ventricular systole,it is an indicator of the onset of ventricular diastole. The A3 eventoccurs during ventricular diastole. As such, the A2 and A3 events may becollectively referred to as ventricular mechanical diastolic eventsbecause they are both indicators of the ventricular diastolic period.

The A4 event 260 is an acceleration signal that occurs during atrialcontraction and active ventricular filling and marks atrial mechanicalsystole. The A4 event 260 is also referred to herein as the “atrialsystolic event” or merely the “atrial event.” The A4 event 260 is theatrial systolic event that is detected from motion sensor signal 250 byatrial event detector circuit 240 for controlling pace timing circuit242 to trigger ventricular pacing pulse delivery by starting an AVpacing interval in response to detecting the A4 event 260 when pacemaker14 is operating in an atrial tracking ventricular pacing mode. Controlcircuit 206 may be configured to detect one or more of the A1, A2, andA3 events from motion sensor signal 250, for at least some ventricularcardiac cycles, for use in positively detecting the A4 event 260 andsetting atrial event detection control parameters. The A1, A2 and/or A3events may be detected and characterized to avoid false detection of A4events and promote reliable A4 event detection for proper timing ofatrial-synchronized ventricular pacing pulses. Various techniques for A4event detection, controlling the AV pacing interval during anatrial-tracking ventricular pacing mode based on A4 events, andswitching between atrial-tracking and non-atrial tracking ventricularpacing mode based on a motion sensor signal are generally disclosed inU.S. patent application Ser. No. 15/280,538 (Splett, et al), U.S. patentapplication Ser. No. 15/280,339 (Sheldon, et al.), U.S. patentapplication Ser. No. 15/366,993 (Sheldon et al.), and U.S. patentapplication Ser. No. 15/377,717 (Demmer et al.), all of which areincorporated herein by reference in their entirety.

As the heart rate increases, the time between the A3 and A4 events maydecrease and the A3 and A4 events may merge and become difficult todistinguish in the motion sensor signal. As a result, detection of theA4 events by RV pacemaker 14 for setting an AV pacing interval toprovide atrial tracking of the ventricular pacing pulses may becomecompromised. RV pacemaker 14 may be configured to switch from the atrialtracking ventricular pacing mode to a non-atrial tracking ventricularpacing mode if the A4 event is not being detected or a condition isdetected that is associated with unreliable A4 event sensing, e.g., ahigh heart rate or high level of patient activity. For example RVpacemaker 14 may switch from a VDDR pacing mode to a VVIR pacing mode inresponse to pacing mode switching criteria being satisfied. During theVVIR pacing mode RV pacemaker 14 may determine a SIR and adjust theventricular LR pacing interval from a base rate interval to a temporaryLR interval set according to the SIR. As the ventricular pacing rate isincreased, however, the ventricular pacing pulse may be delivered duringthe passive or active filling phase of the ventricle, which may cause anundesirable decrease in cardiac output. In order to prevent theventricular pacing pulse from being delivered too early duringventricular filling, a minimum limit of the temporary LR interval may beestablished to limit the maximum rate responsive pacing rate during anon-atrial tracking ventricular pacing mode.

FIG. 5 is a flow chart 300 of one method performed by pacemaker 14 fordetecting the A4 event and controlling ventricular pacing during anatrial tracking pacing mode. At block 302, control circuit 206identifies a ventricular event. The ventricular event may be an R-wavesensed event signal received from sensing circuit 204 or a ventricularpacing pulse delivered by pulse generator 202. Since the ventricular A1,A2 and A3 events may have different characteristics during an intrinsicventricular rhythm than during a ventricular paced rhythm, the methodsdescribed herein for determining amplitudes, time intervals or othercharacteristics of the A1, A2 and A3 events for use in setting A4detection control parameters or confirming A4 event detection may bedetermined for both an intrinsic ventricular rhythm and a ventricularpaced rhythm.

For example, as described in conjunction with the flow charts and timingdiagrams presented herein, various time intervals, sensing windows,atrial refractory period, and atrial event detection threshold amplitudemay be set based on characterizations of one or more of the A1, A2 andA3 events. One set of A4 detection control parameters andcharacteristics of the A1, A2 and A3 events may be determined and storedfor use during episodes of ventricular sensing (ventricular intrinsicrhythm), and another set of A4 detection control parameters andcharacteristics of the A1, A2 and A3 events may be determined and storedfor use during episodes of ventricular pacing.

During ventricular sensing, control circuit 206 may be configured todiscriminate a normal sinus R-wave from a premature ventricularcontraction (PVC) so that ventricular events identified at block 302 foruse in starting a search for the A1 through A4 events from the motionsensor signal do not include PVCs. When a ventricular event, sensed orpaced, is identified at block 302 that is not a PVC, pace timing circuit242 may set an escape interval timer to a ventricular LR pacinginterval. If the LR pacing interval expires (as described below inconjunction with block 315), a ventricular pacing pulse may bedelivered, asynchronous to atrial activity, in order to maintain someminimum, base ventricular rate.

At block 304, atrial event detector 240 detects the A1 through A3 motionsignals. Briefly, atrial event detector 240 may compare the motionsensor signal to one or more predetermined detection thresholdamplitudes during one or more time windows set in response toidentifying the ventricular event at bock 302 for detecting the A1through A3 events. In some examples, the A4 event may also be detectedat block 304 to increase confidence in the positive identification ofeach of the four motion sensor signals A1 through A4 in a given cardiaccycle. In this example, the A1 through A3 events, and optionally A4, maybe detected on a beat-by-beat basis.

After the A1 through A3 events are detected, the A1-A3 time interval isdetermined at block 304 as the time interval from the A1 event detectionto the A3 event detection. The A1-A3 time interval may be used to updatean A1-A3 interval trend at block 308. For example, a running averageA1-A3 time interval may be updated at block 308 using the most recent NA1-A3 time interval measurements, e.g., the most recent three to twelveA1-A3 time intervals.

The A1-A3 time interval is used to set a post-ventricular atrialrefractory period at block 310. This atrial refractory period is alsoreferred to herein as an “A4 refractory period” because A4 eventdetection may be inhibited or ignored during the atrial refractoryperiod in some examples. When a ventricular electrical event isidentified at block 302, atrial event detector 240 may start the atrialrefractory period at block 310. The atrial refractory period may be setbased on the A1-A3 time interval, e.g., to a percentage longer than or afixed interval longer than the A1-A3 time interval. For example, theatrial refractory period may be set to be 50 to 150 ms longer than theA1-A3 time interval, though shorter or longer fixed intervals may beadded to the A1-A3 time interval for setting the atrial refractoryperiod. The fixed time interval used to set the atrial refractory periodmay vary depending on heart rate in some examples.

During the atrial refractory period, any motion sensor events that aredetected, or cross a detection threshold amplitude, are ignored for thepurposes of triggering a ventricular pacing pulse and starting an AVpacing interval. Ventricular mechanical events A1 through A3 may bedetected during the atrial refractory period, as indicated at block 304,to determine the A1-A3 time interval and update the A1-A3 interval trend(blocks 306 and 308), either periodically or on a beat-by-beat basis.

At block 312, atrial event detector circuit 240 monitors the motionsensor signal to detect the A4 event after the expiration of the atrialrefractory period. If the A4 event is not detected before the LR pacinginterval expires (block 315), a ventricular pacing pulse is delivered atblock 316 to ensure a minimum ventricular rate, e.g., at least 40 to 60beats per minute (bpm). Furthermore, it is to be understood that if anintrinsic R-wave is sensed before an A4 event is detected, the processof FIG. 5 may return to block 302 where the sensed R-wave is identifiedas a ventricular electrical event, and control circuit 206 restarts theprocess of detecting the A4 event on the next ventricular cycle.

If the A4 event is detected before the LR pacing interval expires,control circuit 206 sets the AV pacing interval at block 314 in responseto detecting the A4 event. If an intrinsic R-wave is not sensed from thecardiac electrical signal by sensing circuit 204 during the AV pacinginterval, “no” branch of block 316, a ventricular pacing pulse isdelivered by pulse generator 202 at block 318 upon expiration of the AVpacing interval. The ventricular pacing pulse, if delivered, andotherwise the sensed R-wave, is identified as the next ventricular eventat block 302 and the process repeats.

In this way, the A1 through A3 events may be detected from the motionsensor signal on a beat-by-beat (or less frequent) basis for updatingthe A1-A3 time interval trend used to set the atrial refractory periodto provide a high likelihood of positively detecting the A4 event andproperly timing a ventricular pacing pulse in synchrony with the atrialevent. Other motion sensor signal events A1 through A3 are unlikely tobe falsely detected as the A4 event by applying the atrial refractoryperiod set based on the A1-A3 timing.

In some examples, rather than determining an A1-A3 time interval, a timeinterval to the A2 event may be determined so that the atrial refractoryperiod is set based on the A1-A2 time interval to extend through atleast the A2 event and expire before the A3 event. In this example, anA4 detection threshold amplitude may be set higher than an expected A3event amplitude to allow detection of the A4 event earlier in theventricular cycle, for example as the atrial rate is increasing. Inother cases, the time interval from the identified ventricularelectrical event to the A1, A2 or A3 event may be determined and used insetting the atrial refractory period.

In some examples, the process of blocks 304 through 308 is performedperiodically rather than on a beat-by-beat basis. For example detectionof A1-A3 events during the atrial refractory period may occur on everythird cardiac cycle, every eighth cardiac cycle, once per minute, orother predetermined schedule for updating the A1-A3 time interval (orother ventricular event time interval as discussed above) used forsetting the atrial refractory period at block 310. In some cases, theheart rate, paced or intrinsic, may be monitored and the A1-A3 eventsmay be detected for updating the A1-A3 interval trend when the heartrate changes by more than a predetermined amount. For example,ventricular event intervals between consecutive ventricular events maybe determined upon identifying ventricular events at block 302. Theventricular event intervals may be RR intervals between consecutivelysensed intrinsic R-waves or VV intervals between consecutively deliveredventricular pacing pulses and may include RV intervals between a sensedintrinsic R-wave and a consecutively delivered pacing pulse and VRintervals between a delivered pacing pulse and a consecutively sensedR-wave. Both the intrinsic heart rate and the paced rate may change,e.g., when pacemaker 14 is a rate responsive pacemaker. If theventricular event interval changes or a trend in the ventricular eventinterval changes by more than a predetermined amount, the controlcircuit may perform blocks 304 through 308 to update the A1-A3 intervaltrend used for setting the atrial refractory period.

In other examples, if the A4 event is not detected at block 312 afterthe atrial refractory period and before the next ventricular event(intrinsic or paced) is identified at block 302, the control circuit 206may perform the process of blocks 304 through 306 for a predeterminednumber of consecutive or non-consecutive cardiac cycles to update theA1-A3 interval trend used to set the atrial refractory period to restoreA4 detection.

FIG. 6 is an example of motion sensor signals 400 and 410 acquired overtwo different cardiac cycles and obtained within a beating heart. Aventricular pacing pulse is delivered at time 0.0 seconds for bothcardiac cycles. The top sensor signal 400 is received over one cardiaccycle and the bottom sensor signal 401 is received over a differentcardiac cycle. The two signals 400 and 410 are aligned in time at 0.0seconds, the time of the ventricular pacing pulse delivery.

The A1 events 402 and 412 of the respective motion sensor signals 400and 410, which occur during ventricular contraction, are observed to bewell-aligned in time following the ventricular pacing pulse at time 0.0seconds. Similarly, the A2 events 404 and 414 (occurring duringventricular relaxation) and the A3 events 406 and 416 (occurring duringpassive ventricular filling) are well-aligned in time. Since the A1, A2and A3 events are ventricular events, occurring during ventricularcontraction, ventricular relaxation, and passive ventricular filling,respectively, these events are expected to occur at relativelyconsistent intervals following a ventricular electrical event, theventricular pacing pulse in this example, and relative to each other.The time relationship of the A1, A2 and A3 events may be differentfollowing a ventricular pacing pulse compared to following a sensedintrinsic R-wave. However during a stable paced or intrinsic ventricularrhythm the relative timing of A1, A2 and A3 events to each other and theimmediately preceding ventricular electrical event is expected to beconsistent for a given heart rate.

The A4 events 408 and 418 of the first and second motion sensor signals400 and 410 respectively are not aligned in time. The A4 event occursduring atrial systole and as such the time interval of the A4 eventfollowing the immediately preceding ventricular electrical event (sensedR-wave or ventricular pacing pulse) and the preceding A1 through A3events may vary between cardiac cycles.

The consistency of the timing of the A1 through A3 events relative toeach other and the immediately preceding ventricular electrical eventmay be used for determining the atrial refractory period and increasingconfidence in reliably detecting A4 events 408 and 418. The timing ofthe A1 through A3 events, particularly an A3 time interval since aventricular electrical event, may also be used for establishing aminimum temporary LR interval during rate responsive pacing in anon-atrial tracking ventricular pacing mode. In some examples, an A1sensing window 420 may be set based on an expected Vpace-A1 timeinterval. The Vpace-A1 time interval 430 may be measured when the motionsensor signal 400 or 410 crosses an A1 sensing threshold amplitude 440.The A1 sensing window 420 may be adjusted on the next cardiac cyclebased on the Vpace-A1 time interval 430 determined on the currentcardiac cycle or a running average Vpace-A1 time interval.

An A2 sensing window 422 may be set based on an expected Vpace-A2 timeinterval (not explicitly shown but understood to be the total time from0.0 seconds to an A2 event detection) or an A1-A2 time interval 432(time from A1 detection to time of A2 detection). The A2 event 404 or414 may be detected at the time of the first positive-going crossing ofan A2 sensing threshold amplitude 442 by the motion sensor signal 400 or410 during the A2 sensing window 422. The A2 sensing window 422 may beadjusted on the next cardiac cycle based on the Vpace-A2 time intervalor A1-A2 time interval 432 determined on the current cardiac cycle.

Similarly, an A3 sensing window 424 may be set based on an expectedVpace-A3 time interval (not explicitly labeled but understood to be sumof time intervals 430 and 434), A1-A3 time interval 434, or A2-A3 timeinterval (not explicitly labeled but understood to be the time intervalfrom the sensed A2 event 404 or 414 to the sensed A3 event 406 or 416).The A3 event 406 or 416 may be detected during the A3 sensing window 424when the motion sensor signal 400 or 410, respectively, crosses an A3sensing threshold amplitude 444. The threshold crossing resulting in anA3 event detection may be the first positive going threshold crossing,the last negative going threshold crossing (of a rectified signal) oranother fiducial point of the A3 event signal. The A3 sensing window 424may be adjusted on the next cardiac cycle based on the Vpace-A3 timeinterval, A1-A3 time interval 434, or the A2-A3 time interval determinedduring the current cardiac cycle.

Each of the sensing windows 420, 422 and 424 may be set based on ahistory of time intervals determined from a ventricular pacing pulse orsensed intrinsic R-wave to the respective A1 event 402 or 412, A2 event404 or 414 and A3 event 406 or 416 or based on a history of timeintervals between the detected A1, A2 and A3 events or any combinationthereof. For example, the A2 sensing window 422 may be set to startbased on time intervals measured between a ventricular pacing pulse orsensed R-wave and the detected A1 event. The end of the A2 sensingwindow 422 may be set to start based on an A1-A2 time interval 432 orbased on an A1-A3 time interval 434. It is recognized that numerousmethods may be conceived for setting the A1, A2 and A3 sensing windows420, 422 and 424, respectively, based on the consistency of the expectedtime intervals between any combinations of the ventricular electricalevent (paced or sensed) and subsequent A1, A2 and A3 events.Furthermore, it is contemplated that these sensing windows 420, 422 and424 may be set according to different control parameters, such asdifferent fixed time intervals added to or subtracted from measuredevent time intervals depending on whether the ventricular electricalevent is a paced or sensed event and/or depending on heart rate. Theevent time intervals that may be measured and used for setting theonset, offset and duration of the sensing windows 420, 422 and 424 mayinclude any one or combination of the Vpace-A1, Vpace-A2, Vpace-A3,Rsense-A1, Rsense-A2, Rsense-A3, A1-A2, A1-A3, and/or A2-A3 timeintervals determined during a paced and/or intrinsic rhythm.

The sensing threshold amplitudes 440, 442 and 444 may be set uniquelyduring each of the respective sensing windows 420, 422 and 424,respectively, or set to a fixed common value for all sensing windows.The sensing threshold amplitudes 440, 442, and 444 may be fixed ordecaying thresholds and may be automatically adjusted thresholds set tostarting threshold values based on the peak motion sensor signalamplitude detected during each respective window 420, 422 and 424. Themotion sensor signals 400 and 410 are shown as raw signals, but themotion sensor signal may be filtered, amplified and rectified bycircuitry included in motion sensor 212 to provide control circuit 206with a rectified signal that is used to detect the A1 through A4 events.

A post-ventricular, atrial refractory period 436 may be set based on theA1-A3 time interval 434 or based on the sensed R-wave-A3 or Vpace-A3time interval (sum of Vpace-A1 interval 430 and A1-A3 time interval434). In some examples, the atrial refractory period 436 ends upon theexpiration of the A3 sensing window 424. In other examples, the atrialrefractory period 436 ends after the expiration of the A3 sensing window424. The A4 event 408 or 418 may be detected in response to a crossingof an A4 sensing threshold amplitude 446, e.g., the first positive-goingcrossing or a last negative-going crossing, by the rectified motionsensor signal outside the atrial refractory period 436.

In some examples, the A4 detection is confirmed when the A1, A2 and A3events have each been detected during the atrial refractory period 436.If any one of the A1, A2 or A3 events was not detected during the atrialrefractory period 436, the A4 event detection based on a crossing ofthreshold 446 may not be confirmed and not used for starting an AVpacing interval. In other examples, at least one of the A1, A2 or A3events may be required to be detected during a respective sensing window420, 422, or 424 on a beat-by-beat basis for confirming an A4 detectionafter the atrial refractory period 436.

The A1, A2 and/or A3 events sensed during the respective A1 sensingwindow 420, A2 sensing window 422 and A3 sensing window 424 may be usedfor updating the atrial refractory period 436 as described inconjunction with FIG. 5 on a beat-by-beat or less frequent basis withoutrequiring positive detection of each of A1, A2, and/or A3 for confirmingan A4 detection on each beat. Setting the atrial refractory period basedon detection and relative timing of the A1 through A3 events enables theatrial refractory period to be set based on the consistent timing of theventricular motion sensor signal events so that A4 events may bedetected with high reliability even when the timing of the A4 eventrelative to the A1-A3 events and the preceding ventricular electricalevent is variable. A3 event detection may additionally or alternativelybe used for determining an A3 time interval and establishing a minimumLR pacing interval used during the non-atrial tracking ventricularpacing mode. The A3 time interval may be determined during the atrialtracking ventricular pacing mode and used during the non-atrial trackingventricular pacing mode for limiting the minimum temporary LR pacinginterval during rate responsive pacing.

In some examples, an A3-A4 event interval 445 is determined and used foradjusting the A4 refractory period 436. For example, the A3 event 406may be detected by detecting the last, negative-going crossing of an A3event detection threshold amplitude 444 during the A3 sensing window424. The A4 event 408 may be detected by the first positive-goingcrossing of the A4 event detection threshold amplitude 446 after theexpiration of the A4 refractory period 436. The A3-A4 event interval 445is determined as the time from the A3 event detection and the time ofthe A4 event detection. This A3-A4 event time may be compared to aprevious A3-A4 event time, e.g., compared to one or more preceding A3-A4event times which may be determined during the respective one or morepreceding cardiac cycles or to a running average A3-A4 event timedetermined from two to five or other predetermined number of previouslydetermined A3-A4 event times. If a change in the A3-A4 event timeinterval compared to one or more preceding A3-A4 event times isdetected, the A4 refractory period 436 may be adjusted. As the A3-A4event time is detected to shorten or increase, the control circuit 206may decrease or lengthen the A4 refractory period, respectively, toaccount for changes in the time interval between the ventriculardiastolic event and the atrial systolic event as the atrial ratechanges.

FIG. 7 is an averaged motion sensor signal 500 that may be determined bycontrol circuit 206 by averaging the motion sensor signal obtained overmultiple cardiac cycles, e.g., signals 400 and 410 of FIG. 6. Theaveraged motion sensor signal 500 may represent the average of 3 to 20or other predetermined number of cardiac cycles. The raw motion sensorsignal or a filtered, amplified and/or rectified motion sensor signalmay be buffered beginning from a ventricular electrical event, pacingpulse or sensed R-wave, at time 0.0 seconds until the next ventricularelectrical event. The buffered motion sensor signal obtained over onecardiac cycle may be averaged with the buffered motion sensor signalsobtained over a predetermined number of other cardiac cycles to produceaveraged motion sensor signal 500.

A ventricular electrical signal 510 is shown aligned in time withaveraged motion sensor signal 500. Ventricular electrical signal 510 maybe passed from sensing circuit 204 to control module 206 and includes anR-wave 512, which may be an evoked or intrinsic R-wave, and a T-wave514. R-wave 512 is followed by the ventricular contraction A1 event 502.The ventricular relaxation A2 event 504 occurs during T-wave 514. Thepassive ventricular filling A3 event 506 occurs after T-wave 514.

Since the A1, A2 and A3 events are ventricular mechanical events, theyoccur at consistent time intervals relative to each other and relativeto ventricular electrical events (R-wave 512 and T-wave 514). As aresult, the signal-to-noise ratio of the A1 signal 502, A2 signal 504and A3 signal 506 is improved in the averaged motion sensor signal 500compared to the single-cycle motion sensor signals 400 and 410 of FIG.6. The averaged A1 event 502, A2 event 504 and A3 event 506 have animproved signal-to-noise ratio compared to the A1, A2 and A3 eventsobserved in the motion sensor signal 400 or 410 of a single cardiaccycle as shown in FIG. 6, making A1, A2, and A3 event detection from theaveraged motion signal 500 more reliable.

A single event detection threshold amplitude 540 may be defined suchthat the a crossing of the threshold 540 by the averaged, rectifiedmotion sensor signal 500 within the A1 sensing window 520, A2 sensingwindow 522 and A3 sensing window 524 is detected as the respective A1event 502, A2 event 504, and A3 event 506. The threshold crossing may bea first, positive-going crossing or a last, negative-going crossing invarious examples. Alternatively, unique detection threshold amplitudesmay be defined for each sensing window 520, 522 and 524 for detectingthe respective A1, A2 and A3 events. The sensing windows 520, 522 and524 may be initially set according to expected A1, A2 and A3 eventtiming following the ventricular pacing pulse or R-wave 512 and may beadjusted according to the actual detection time of each respective A1event 502, A2 event 504, and A3 event 506 based on a threshold crossing.The sensing windows 520, 522 and 524 may be set based on ventricularpacing rate or atrial event rate, e.g., based on A4-A4 event intervals.The sensing windows 520, 522 and 524 may also be set differentlyfollowing a ventricular pacing pulse than following an intrinsic R-wavesensed event since the timing of the A1, A2 and A3 events and T-wave 514may be altered during ventricular pacing compared to during an intrinsicventricular rhythm.

The atrial systolic A4 event timing, which is independent of theventricular electrical event timing, may be more variable from onecardiac cycle to the next with respect to the ventricular electrical andmechanical events, e.g., as shown by the relative timing of the A4events 408 and 418 of signals 400 and 410 (FIG. 6). As a result, the A4signal is largely attenuated in the averaged motion signal 500 in FIG.7. The improved signal-to-noise ratio of the A1 through A3 events andattenuation of the A4 event in the averaged motion signal 500 enablescontrol circuit 206 to reliably detect the signal averaged A1 event 502,A2 event 504 and A3 event 506 for determining one or more ventricularevent time intervals for use in setting A1, A2 and A3 detection windows420, 422, and 424, respectively, setting detection threshold amplitudesfor detecting the A1, A2, A3 and/or A4 events, and/or setting atrialrefractory period 436 used on a beat-by-beat basis for A4 eventdetection as shown in FIG. 6.

For example, a ventricular R-wave or pacing pulse to A1 time interval530, an A1-A3 time interval 534, A1-A2 time interval 536, a ventricularR-wave or pacing pulse to A3 time interval 516, and/or a T-wave to A3time interval 518 may be determined by control circuit 206 from theaveraged motion signal 500 and the cardiac electrical signal 510. Theatrial refractory period 436 is started upon delivering a ventricularpacing pulse or sensing an intrinsic R-wave. The atrial refractoryperiod 436 may be set to expire after a predetermined time interval,e.g., 30 to 100 ms, after the A3 time interval 516. For instance, iftime interval 516 is 700 ms, the atrial refractory period 436 may be setto expire 750 ms after the ventricular pacing pulse or sensed R-wavethat started the atrial refractory period. Instead of using a timeinterval ending with the A3 event detection, a time interval ending withthe A2 event detection may be determined and used in controlling theduration of the atrial refractory period 436. As described above, the A2event, which occurs during T-wave 514, is an indicator of the end ofventricular mechanical systole and the onset of ventricular mechanicaldiastole. The A3 event occurs during ventricular mechanical diastole,during the passive ventricular filling phase. As such the timing of theA2 event 504 or the timing of the A3 event 506 relative to anotherventricular electrical event (ventricular pacing pulse, R-wave 512, orT-wave 514) may be used for controlling the duration and expiration timeof atrial refractory period 436. In other words, the timing of aventricular mechanical diastolic event, A2 event 504 or A3 event 506,may be determined and used to set the atrial refractory period 436 thatis applied on a beat-by-beat basis for detecting A4 events.

The T-wave 514 may be sensed by sensing circuit 206 on a beat-by-beatbasis by control circuit 206 or by sensing circuit 204 from cardiacelectrical signal 510. The T-wave 514 may be sensed at a maximum peakamplitude of a rectified cardiac electrical signal or a maximum absolutepeak amplitude in a non-rectified cardiac signal received by controlcircuit 206 from sensing circuit 204. Alternatively, T-wave 514 may besensed by sensing circuit 204 in response to the cardiac electricalsignal crossing a T-wave sensing threshold amplitude after theventricular pacing pulse or R-wave sensed event signal. In some cases, aT-wave sensing window may be applied after the R-wave sensed eventsignal or a delivered pacing pulse to facilitate T-wave sensing.

The T-wave 514 may be sensed during the atrial refractory period 436.Control circuit 206 may terminate the atrial refractory period 436 at apredetermined time interval after sensing T-wave 514. For instance ifthe T-wave to A3 time interval 518 is determined to be 150 ms from theaveraged motion signal 500, control circuit 206 may terminate the atrialrefractory period 436 at 180 ms after sensing the T-wave to promotereliable sensing of the A4 event.

Atrial event detector circuit 240 may be a processor-based circuit thatdetermines the averaged motion sensor signal 500 over multiple cardiaccycles, detects A1, A2 and A3 events 502, 504, and 506 from the averagedmotion sensor signal 500, and sets the atrial refractory period 436based on the timing of at least one ventricular mechanical diastolicevent, e.g., the A3 event 506, detected from the average motion sensorsignal 500. In other examples, the A2 event is used as a ventriculardiastolic mechanical event for marking the approximate timing of theonset of ventricular diastole. The A4 event, e.g., event 408 or 418(FIG. 6) may be detected on a beat-by-beat basis from the non-averagedmotion sensor signal after the atrial refractory period 436 expires. TheA3 event 506 may be used to determine an A3 time interval following theA1 event or following a ventricular electrical event, e.g., a sensedR-wave or delivered ventricular pacing pulse. Control circuit 206 mayuse the A3 time interval determined from the ensemble averaged motionsignal for setting a minimum temporary LR interval used for controllingventricular pacing rate during the non-atrial tracking ventricularpacing mode.

FIG. 8 is a flow chart 600 of a method performed by pacemaker 14 fordetecting atrial events and controlling atrial-synchronized ventricularpacing during an atrial tracking pacing mode according to one example.At block 602, control circuit 206 identifies a ventricular electricalevent, which may be an intrinsic R-wave sensed by sensing circuit 204 ora ventricular pacing pulse delivered by pulse generator 202. A LR pacinginterval may be set at block 602 upon identifying the ventricularelectrical event, as described in conjunction with FIG. 5, in order tomaintain a minimum, base ventricular rate in the absence of A4 eventdetections.

At block 604, the motion sensor signal is buffered over the cardiaccycle, e.g., until the next ventricular electrical event is identified.At block 606, the buffered motion signal is averaged with bufferedmotion sensor signals acquired over a predetermined number of cardiaccycles to obtain an averaged motion signal with improved A1, A2 and A3signal-to-noise ratio and attenuated A4 signal compared to thenon-averaged motion sensor signal.

At block 608 the A1-A3 time interval or a ventricular electrical eventto A3 time interval is determined from the averaged motion sensor signalby detecting the signal averaged A1, A2 and A3 events as described abovein conjunction with FIG. 7. The A3 time interval is used to set theatrial refractory period at block 610 by atrial event detector circuit240. As described above, the atrial refractory period may be set apredetermined percentage or fixed time interval longer than the A1-A3time interval or a ventricular electrical event to A3 time interval orset to expire upon expiration of an A3 sensing window that is definedbased on relative timing of the A1, A2, and A3 events. In otherexamples, an A2 time interval is determined at block 608 for use insetting the A4 refractory period. The A2 and A3 events are ventricularmechanical diastolic event markers that may be used for controlling thetiming of the expiration of the A4 refractory period to occur near thestart or during the ventricular passive filling phase, before the activeventricular filling phase associated with atrial systole.

The atrial refractory period is started at block 610 upon identifyingthe ventricular electrical event at block 602. In some examples, signalaveraging and determination of the A3 time interval (or A2 timeinterval) for setting the atrial refractory period may occur on abeat-by-beat basis using an averaged motion signal. In other examples,the A3 time interval is determined periodically or in response to achange in the atrial rate, e.g., determined from A4-A4 intervals, or achange between a sensed and paced ventricular rhythm. The most recentlyupdated A3 time interval (or A2 time interval) determined from theaveraged motion sensor signal may be used to set the atrial refractoryperiod at block 610. The expiration of the atrial refractory period maybe set on the fly during an already started atrial refractory periodbased on the A3 time interval determined during the current ventricularcycle. In other examples, the A3 time interval determined on a precedingventricular cycle is used to set the atrial refractory period for thecurrent ventricular cycle so that the atrial refractory period endsduring or after an expected time of the A3 event, or in some cases priorto an expected A3 event but after an expected A2 event.

In other examples, the duration of the atrial refractory period may becontrolled on a beat-by-beat basis by starting the atrial refractoryperiod upon the identified ventricular event, sensing the T-wave duringthe atrial refractory period, and terminating the atrial refractoryperiod a predetermined time interval after the sensed T-wave, where thepredetermined time interval is based on the T-wave to A3 time interval518 determined from the averaged motion signal 500 (FIG. 7).

If an A4 event is detected from the non-averaged motion sensor signal atblock 612, after the atrial refractory period expires, an AV pacinginterval is set at block 614. The A4 event may be detected based on anA4 detection threshold amplitude crossing by the raw motion sensorsignal or by the rectified signal. The pace timing circuit 242 sets anAV pacing interval at block 614 in response to the detected A4 signal.If an intrinsic R-wave is not sensed before expiration of the AV pacinginterval, as determined at block 616, the scheduled ventricular pacingpulse is delivered at block 620. In some cases, the A4 event may not bedetected before a lower rate pacing interval expires at block 615. Anatrial-asynchronous ventricular pacing pulse may be delivered at block620 if the lower rate pacing interval expires before an A4 event isdetected to maintain a programmed ventricular base rate, causing theprocess to return to block 602 where the ventricular pacing pulse isidentified as the next ventricular electrical event.

FIG. 9 is a flow chart 650 of a method for controlling ventricularpacing mode by pacemaker 14 according to one example. Control circuit206 is configured to control pacemaker 14 to operate in a first pacingmode, either an atrial-tracking pacing mode or a non-atrial trackingpacing mode for controlling ventricular pacing pulses and to determinewhen to switch to a second pacing mode, the other one of theatrial-tracking pacing mode or the non-atrial tracking pacing mode,based on an analysis of the motion sensor signal.

As shown in FIG. 9, ventricular pacing pulses are delivered according tothe selected pacing mode at block 652. For example, the pacemaker 14 maybe operating in an atrial-tracking VDD pacing mode during whichventricular pacing pulses are scheduled at an AV pacing interval inresponse to detecting an A4 event as described above. In the absence ofa detected A4 event, a ventricular pacing pulse is delivered at a VDD LRinterval when an intrinsic R-wave is not detected before expiration ofthe LR interval to prevent ventricular asystole. At other times, thepacemaker 14 may be operating in a non-atrial tracking VVIR or VDIRpacing mode during which pacing pulses are scheduled at a ventricular LRpacing interval, which may be a permanent VVIR or VDIR LR intervalcorresponding to a base pacing rate or a temporary LR interval set basedon a patient physical activity metric determined from the motion sensorsignal (or other sensor signal correlated to patient metabolic demand).

During operation in the selected ventricular pacing mode, the controlcircuit 206 is configured to determine one or more motion signal metricsfrom the motion signal at block 654. The one or more motion signalmetrics are compared to first pacing mode switching criteria at block656. If the first pacing mode switching criteria are met, the controlcircuit 206 switches from the current, first pacing mode, either anatrial tracking pacing mode or a non-atrial tracking pacing mode, to asecond pacing mode. The second pacing mode is the other one of theatrial tracking pacing mode or the non-atrial tracking pacing modedepending on which pacing mode the control circuit 206 is switchingfrom.

During the second pacing mode, the control circuit 206 determines one ormore motion signal metrics at block 654, which may be the same motionsignal metric(s) or different motion signal metric(s) that weredetermined during the first pacing mode. Control circuit 206 determinesif second pacing mode switching criteria are met by the motion signalmetric(s) during the second pacing mode and switches back to the firstpacing mode in response to the second pacing mode switching criteriabeing met.

Among the motion signal metrics that may be determined at block 654 aremetrics that are correlated to patient physical activity and/orcorrelated to loss of reliable A4 event detection. Motion signal metricscorrelated to patient physical activity may be a patient activity metriccorrelated to patient metabolic demand such as an activity count or asensor indicated pacing rate determined from the patient activitymetric. A patient posture may be determined from the motion signal as anindirect indicator of patient activity, e.g., by detecting a non-uprightposture as a resting or inactive posture and detecting an uprightposture as a non-resting or active posture. Patient posture may also bedetermined as a motion signal metric that is correlated to loss ofreliable A4 event detection since the patient posture may influence theA4 signal strength and reliability of A4 event detection. For example, aparticular patient posture, for instance a left side-lying posture orother identified posture, may be determined to confound A4 eventdetection in an individual patient and be criteria for pacing modeswitching.

A motion signal metric that may be determined at block 654 that iscorrelated to loss of reliable A4 event detection may be a count ofventricular cycles that occur without a detected A4 event. Anothermotion signal metric that may be correlated to a loss of reliable A4event detection may be the A3-A4 time interval. Shortening of the A3-A4time interval, e.g., interval 445 in FIG. 6, or another ventricularevent to A4 time interval, may indicate an increasing atrial rate thatmay lead to fusion of the A3 and A4 events and/or be associated withincreased patient physical activity, both of which may confound A4 eventdetections.

A motion signal metric that may be determined at block 654 that iscorrelated to both patient physical activity and loss of reliable A4event detection may be determined by processing motion signal samplepoints acquired over a predetermined time segment of the motion signal.For example, the average amplitude of all sample points during thepredetermined time segment, a summation of the sample point amplitudesduring predetermined time segment, the mean frequency during the timesegment, mean slope, or other metric of the motion signal energy orentropy during the entirety of the predetermined time segment mayindicate an increase in patient physical, non-cardiac motion, signalnoise and/or a potential decrease in the reliability of A4 eventdetection due to increased motion signal peaks present in the motionsignal. The time segment for determining the motion signal metric may beone cardiac cycle or portion thereof or may be independent of cardiaccycle timing, e.g., any 500 ms, 1 second, 2 second or otherpredetermined time segment or multiple predetermined time segments ofthe motion sensor signal.

Accordingly, the one or more motion signal metrics determined at block654 may include direct or indirect metrics of non-cardiac, patientphysical motion and/or A4 event detection reliability. The motion signalmetrics determined from the motion sensor signal included in pacemaker14 may include metrics of patient physical activity; patient bodyposture; A4 event detection counts; cardiac mechanical event (A1, A2,A3, and/or A4) time intervals, amplitudes and/or other features; andmotion signal metrics determined by processing the motion signal samplepoints over a predetermined time segment to obtain a metric correlatedto the motion signal energy and/or entropy over the entirety of the timesegment. Methods for controlling pacing mode switching between an atrialtracking ventricular pacing mode and a non-atrial tracking ventricularpacing mode are disclosed in the above-incorporated U.S. patentapplication Ser. No. 15/366,993 (Sheldon, et al.).

FIG. 10 is a flow chart 700 of a method performed by RV pacemaker 14 forestablishing a minimum ventricular LR interval according to one example.At block 702, RV pacemaker 14 identifies a ventricular systolic event.The ventricular systolic event may be a ventricular pacing pulsedelivered by pulse generator 202, an intrinsic R-wave sensed by sensingcircuit 204, or a sensed A1 event detected from the motion sensor signalby control circuit 206.

At block 704, control circuit 206 detects the A3 event from the motionsensor signal. The A3 event is detected from the motion signal producedby motion sensor 212 enclosed within the housing 150 of pacemaker 14,which is implanted within the ventricular chamber of a beating heart. Asdescribed above, the housing 150 is configured to be implanted whollywithin a ventricular heart chamber, e.g., by having a size and shapethat is deliverable by a transvenous catheter. Housing 150 may have agenerally cylindrical shape with an outer diameter that is 10 mm orless, 8 mm or less, 6 mm or less, or approximately 20 French toaccommodate implantation wholly within the heart. The A3 event detectedfrom the intra-ventricular motion signal marks the time of passiveventricular filling during which ventricular pacing pulse delivery isundesirable. If a ventricular pacing pulse is delivered during thepassive filling phase, incomplete filling of the ventricle may lead todeleterious hemodynamic consequences. After detecting the A3 event, anA3 time interval is determined at block 706 as the time interval fromthe identified ventricular systolic event to the detected A3 event. TheA3 time interval may be determined for multiple cardiac cycles todetermine a median, mean or other statistical measure of centeredness,maximum or range of the A3 time interval. In some examples, the motionsensor signal is averaged over multiple cardiac cycles and the A3interval is determined from the averaged motion sensor signal, which mayhave improved signal to noise ratio of ventricular A1, A2 and A3 eventscompared to the motion sensor signal over a signal cardiac cycle. The A3time interval determined at block 706 may be the updated A1-A3 timeinterval trend determined at block 708 in FIG. 5 that is used to set theA4 refractory period during the atrial tracking ventricular pacing mode.The A3 time interval determined at block 706 may alternatively bedetermined as the A3 time interval determined from the ensemble averagedmotion signal at block 608 of FIG. 8 during the atrial trackingventricular pacing mode. The A3 time interval may be determined during aresting state of the patient, e.g., when the SIR indicates a pacing ratethat is not greater than the base pacing rate, or during non-strenuousactivity, e.g., when the SIR is not greater than an activities of dailyliving (ADL) rate.

Pacemaker 14 may be operating in the atrial tracking ventricular pacingmode at the time that the A3 time interval is determined sinceconditions for pacing mode switching to the non-atrial trackingventricular pacing mode may not be satisfied when the patient is restingor engaged in non-strenuous activity. In some cases, however, the A3time interval may be determined during the non-atrial trackingventricular pacing mode if pacing mode switching criteria have been met.For instance, if a particular patient posture has been detected that iscorrelated to unreliable A4 event sensing, pacemaker 14 may be operatingin a VVIR pacing mode when the process of FIG. 10 is performed. The A3time interval, however, may be determined during the non-atrial trackingventricular pacing mode when the pacing rate or sensed ventricular rateis below a predetermined threshold or up to the ADL rate, for example.

If pacemaker 14 is delivering ventricular pacing pulses during thedetermination of the A3 time interval, the Vpace-A3 time interval may bedetermined at block 706. This A3 time interval is an estimate of thetime from a delivered ventricular pacing pulse until the ventricularpassive filling phase and provides a basis for a minimum pacing intervalduring rate responsive, non-atrial tracking ventricular pacing. Ifventricular pacing is not occurring during A3 time intervaldetermination, an R-A3 or an A1-A3 time interval may be determined atblock 706. The R-A3 time interval represents the time interval from theintrinsic R-wave to passive filling during a non-paced ventricularrhythm. The A1-A3 time interval represents the time interval from the A1event to the A3 event of the motion sensor signal. The R-A3 timeinterval may be different than the Vpace A3 time interval since the timefrom the ventricular pacing pulse until the electrical depolarization ofthe myocardium is not included in the R-A3 time interval and theconduction time through the ventricles may be different during anintrinsic beat than during a paced beat. Likewise, the A1-A3 timeinterval is expected to be different than the Vpace-A3 time intervalsince the time from the ventricular pacing pulse until the mechanicalsystole corresponding to the A1 event is not included in the A1-A3 timeinterval. If ventricular pacing is not occurring during A3 time intervaldetermination at block 706 such that the Vpace-A3 time interval is notavailable for measurement, the R-A3 or A1-A3 time interval may bedetermined and adjusted to better approximate a Vpace-A3 time intervalthat takes into account a delay from the time of a pacing pulse deliveryuntil electrical depolarization or mechanical activation.

At block 708, control circuit 206 establishes a minimum LR interval forlimiting the maximum pacing rate during a rate responsive, non-atrialtracking ventricular pacing mode. The minimum LR interval is based onthe A3 time interval determined at block 706. In one example, theminimum LR interval may be set as a percentage of the A3 time interval,e.g., 110% to 120% of the A3 time interval or a fixed time intervaladjustment may be applied to the A3 time interval, e.g., 10 to 50 ms maybe added to the A3 time interval. The minimum LR interval sets a rateresponsive pacing rate maximum rate limit for promoting completion of atleast the passive ventricular filling phase prior to pacing pulsedelivery during increased physical activity of the patient.

In some examples, the A3 time interval may be determined for multipleheart rates at block 706 to generate a table or function of the A3 timeinterval dependent on heart rate. Since the A3 time interval may changewith heart rate, A3 time intervals may be binned for several cardiaccycles after the ventricular rate (paced or intrinsic) has been stablefor a predetermined number of cycles. The resting A3 time interval maybe used to set the minimum LR interval at block 708. In other examples,the A3 time interval for higher heart rates, at which A4 event detectionmay become compromised, may be determined or extrapolated from binned A3time intervals determined for multiple heart rates and used to set theminimum LR interval at block 708. The A3 time interval may be determinedfor multiple heart rates during an atrial-tracking pacing mode when theA3 and A4 events are being reliably detected. A corrected A3 timeinterval may be extrapolated for a heart rate greater than the pluralityof heart rates and used to establish the minimum pacing rate intervalbased on the corrected A3 time interval.

FIG. 11 is a flow chart 800 of one method performed by RV pacemaker 14for controlling ventricular pacing according to one example. At block802, pacemaker 14 is initially operating in the atrial trackingventricular pacing mode, e.g. as described above in conjunction withFIG. 8. During the atrial tracking pacing mode, pacemaker 14 establishesa minimum LR interval at block 804. The minimum LR interval may beestablished based on an A3 time interval using any of the techniquesdescribed above. The minimum LR interval defines the shortestventricular pacing interval corresponding to a maximum pacing rate thatcan be delivered during a rate responsive, non-atrial trackingventricular pacing mode. The minimum LR interval may be establishedwhile the patient is at rest or engaged in sub-maximal or relatively lowlevel of physical activity (e.g., ADL) and may be based on a Vpace-A3,R-A3, or A1-A3 time interval as described above. In this way, theminimum LR interval may be established at a time that the A3 and A4events are reliably detectable, before fusion of A3 and A4 events whichmay occur during higher heart rates, e.g., associated with increasedpatient activity.

During the atrial tracking pacing mode, control circuit 206 detects A4events at block 806 and sets an AV pacing interval in response todetecting an A4 event for scheduling atrial-synchronized ventricularpacing pulses. If an intrinsic R-wave is not sensed, control circuit 206controls pulse generator 202 to deliver a ventricular pacing pulse atthe expiration of the AV interval. In some examples, control circuit 206may set an A4 refractory period in response to a ventricular event,sensed or paced, during which A4 event detection is inhibited orignored. The A4 refractory period may be based on a previously measuredA3 time interval, e.g., to a percentage longer than or a fixed intervallonger than the A1-A3 time interval. During the atrial refractoryperiod, any motion sensor events that are detected may be ignored forthe purposes of triggering a ventricular pacing pulse and starting an AVpacing interval. Ventricular mechanical events A1 through A3 may bedetected during the atrial refractory period to update the A4 refractoryperiod and update the A3 time interval.

At block 808, control circuit 206 determines if pacing mode switchingcriteria are met. Control circuit 206 may determine one or more motionsignal metrics at block 808 and compare the metric(s) to pacing modeswitching criteria. Among the motion signal metrics that may bedetermined at block 808 are metrics that are correlated to patientphysical activity and/or correlated to loss of reliable A4 eventdetection. Motion signal metrics correlated to patient physical activitymay be a patient activity metric correlated to patient metabolic demandsuch as an activity count or a SIR determined from the patient activitymetric. A patient posture may be determined from the motion signal as anindirect indicator of patient activity, e.g., by detecting a non-uprightposture as a resting or inactive posture and detecting an uprightposture as a non-resting or active posture. Patient posture may also bedetermined as a motion signal metric that is correlated to loss ofreliable A4 event detection since the patient posture may influence theA4 signal strength and reliability of A4 event detection. For example, aparticular patient posture may be determined to confound A4 eventdetection in an individual patient and be criteria for pacing modeswitching.

A motion signal metric that may be determined at block 808 that iscorrelated to loss of reliable A4 event detection may be patientactivity, patient posture, or a count of ventricular cycles that occurwithout a detected A4 event. Another motion signal metric that may becorrelated to a loss of reliable A4 event detection may be the A3-A4time interval. Shortening of the time interval between a detected A3 anda detected A4 event, or another ventricular event to A4 time interval,may indicate an increasing atrial rate that may lead to fusion of the A3and A4 events and/or be associated with increased patient physicalactivity, both of which may confound A4 event detection. Techniquesgenerally disclosed in the above-incorporated '993 application forcontrolling ventricular pacing mode switching from an atrial trackingpacing mode to a non-atrial tracking pacing mode based on motion sensorsignal metric(s) may be used at block 808. The mode switching criteriamay be defined to promote switching to non-atrial tracking ventricularpacing when A4 events are or at risk of being under-detected andventricular rate support is needed. The mode switching criteria may bedefined to preferentially provide atrial synchronized ventricular pacingwhen A4 events are being detected or when A4 events are not beingdetected but ventricular pacing at the programmed lower rate is adequatefor supporting the patient's physical activity level, e.g., a restingstate.

If mode switching criteria are met at block 808, control circuit 206switches to the non-atrial tracking ventricular pacing mode at block810. The non-atrial tracking pacing mode may be a rate responsive modeto provide ventricular rate support during periods of increased patientactivity. As such, control circuit 206 may determine a SIR from themotion sensor signal at block 812. Methods for determining a SIR aregenerally described in the above-incorporated references. Techniques forlimiting a maximum rate response pacing rate using the A3 time intervalas disclosed herein are not limited to a particular method fordetermining the SIR. Control circuit 206 determines the ventricularpacing interval corresponding to the SIR at block 814 and compares thisSIR interval to the minimum LR interval previously established duringthe atrial tracking pacing mode (at block 804). If the SIR interval isequal to or greater than the minimum LR interval, control circuit 206controls pulse generator 204 to schedule ventricular pacing pulses at atemporary LR interval set equal to the SIR interval at block 818.

If the SIR interval is less than minimum LR interval, pacing at the SIRmay cause ventricular pacing pulses to be delivered during passivefilling of the ventricle. In response to the SIR interval being lessthan the minimum LR interval at block 814, control circuit 206 controlsthe pulse generator 202 to deliver ventricular pacing pulses a rateresponse limit defined by the minimum LR interval. For example, the A3time interval determined during the atrial tracking pacing mode may be400 ms. A minimum LR interval may be set to 20 ms greater than the A3time interval, 420 ms in this example. The minimum LR interval of 420 mscorresponds to a rate response limit of 136 pacing pulses per minute. Ifthe SIR determined at block 412 based on a patient activity metricderived from the motion sensor signal is greater than 136 pulses perminute, control circuit 206 controls pulse generator 202 to schedulepacing pulses at the maximum rate response limit of 136 pulses perminute at block 816 (at the minimum LR interval of 420 ms). As long asthe SIR interval is equal to or longer than the minimum LR interval,control circuit 206 schedules ventricular pacing pulses at the SIRinterval and controls pulse generator 202 to deliver pacing pulses atthe SIR at block 818.

It should be understood that, depending on the example, certain acts orevents of any of the methods described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of themethod). Moreover, in certain examples, acts or events may be performedconcurrently, e.g., through multi-threaded processing, interruptprocessing, or multiple processors, rather than sequentially. Inaddition, while certain aspects of this disclosure are described asbeing performed by a single circuit or unit for purposes of clarity, itshould be understood that the techniques of this disclosure may beperformed by a combination of units or circuits associated with, forexample, a medical device.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored as one or more instructions orcode on a computer-readable medium and executed by a hardware-basedprocessing unit. Computer-readable media may include computer-readablestorage media, which corresponds to a tangible medium such as datastorage media (e.g., RAM, ROM, EEPROM, flash memory, or any other mediumthat can be used to store desired program code in the form ofinstructions or data structures and that can be accessed by a computer).

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

Thus, a pacemaker has been presented in the foregoing description withreference to specific examples. It is to be understood that variousaspects disclosed herein may be combined in different combinations thanthe specific combinations presented in the accompanying drawings. It isappreciated that various modifications to the referenced examples may bemade without departing from the scope of the disclosure and thefollowing claims.

What is claimed is:
 1. An intracardiac ventricular pacemaker,comprising: a pulse generator configured to generate and deliver pacingpulses to a ventricle of a patient's heart via electrodes coupled to thepacemaker; a motion sensor configured to produce a motion signal; acontrol circuit coupled to the motion sensor and the pulse generator;and a housing wholly implantable within a ventricular chamber of thepatient's heart and enclosing the pulse generator, the motion sensor andthe control circuit; the control circuit being configured to: identify aventricular systolic event; detect a ventricular passive filling eventsignal from the motion signal; determine a time interval from theventricular systolic event to the ventricular passive filling event;establish a minimum pacing interval based on the time interval from theventricular systolic event to the ventricular passive filling event, theminimum pacing interval corresponding to a maximum ventricular pacingrate; determine a sensor indicated pacing rate interval; compare thesensor indicated pacing rate interval to the minimum pacing interval;determine that the sensor indicated pacing rate interval is less thanthe minimum pacing interval; and control the pulse generator to delivera non-atrial tracking ventricular pacing pulse at the minimum pacinginterval corresponding to the maximum ventricular pacing rate inresponse to the sensor indicated pacing rate interval being less thanthe minimum pacing interval.
 2. The pacemaker of claim 1, wherein thecontrol circuit is configured to: control the pulse generator to deliverventricular pacing pulses in an atrial tracking pacing mode by:detecting an atrial systolic event from the motion signal, setting anatrioventricular pacing interval in response to detecting the atrialsystolic event, and controlling the pulse generator to deliver aventricular pacing pulse upon expiration of the atrioventricular pacinginterval; and determine the time interval from the ventricular systolicevent to the ventricular passive filling event during the atrialtracking pacing mode.
 3. The pacemaker of claim 2, wherein the controlcircuit is further configured to: determine that pacing mode switchingcriteria are satisfied; switch from controlling the pulse generator inthe atrial tracking pacing mode to controlling the pulse generator in anon-atrial tracking pacing mode in response to the pacing mode switchingcriteria being satisfied; and determine the sensor indicated rate duringthe non-atrial tracking pacing mode.
 4. The pacemaker of claim 2,wherein the control circuit is further configured to: set an atrialrefractory period in response to identifying the ventricular systolicevent; detect the ventricular passive filling event from the motionsignal during the atrial refractory period; and detect the atrialsystolic event outside the atrial refractory period.
 5. The pacemaker ofclaim 1, wherein the control circuit is configured to: detect a restingstable heart rate; and determine the time interval from the ventricularsystolic event to the ventricular passive filling event during theresting stable heart rate.
 6. The pacemaker of claim 1, wherein thecontrol circuit is configured to: determine the time interval from theventricular systolic event to the ventricular passive filling event foreach of a plurality of heart rates; extrapolate a corrected timeinterval from the ventricular systolic event to the ventricular passivefilling event for a heart rate greater than the plurality of heartrates; and establish the minimum pacing rate interval based on thecorrected time interval.
 7. The pacemaker of claim 1, wherein thecontrol circuit is further configured to: determine a corrected timeinterval based on a function of the time interval from the ventricularsystolic event to the ventricular passive filling event and a targetheart rate interval; and establish the minimum pacing rate intervalbased on the corrected time interval.
 8. The pacemaker of claim 1,wherein the control circuit is configured to identify the ventricularsystolic event from the motion signal.
 9. The pacemaker of claim 1,further comprising a sensing circuit configured to receive a cardiacelectrical signal and sense an R-wave from the cardiac electricalsignal, wherein the control circuit is configured to identify theventricular systolic event as one of a ventricular pacing pulse or thesensed R-wave.
 10. The pacemaker of claim 1, wherein the control circuitis further configured to: determine an ensemble averaged motion signalfrom the motion signal acquired over a plurality of cardiac cycles; anddetermine the time interval from the ventricular systolic event to theventricular passive filling event from the ensemble averaged motionsignal.
 11. The pacemaker of claim 1, wherein the control circuit isconfigured to: control the pulse generator to deliver ventricular pacingpulses in an atrial tracking pacing mode by: determining an ensembleaveraged motion signal from the motion signal acquired over a pluralityof cardiac cycles; determining the time interval from the ventricularsystolic event to the ventricular passive filling event from theensemble averaged motion signal; setting an atrial refractory periodbased on the time interval; detecting an atrial systolic event from themotion signal outside the atrial refractory period; setting anatrioventricular pacing interval in response to detecting the atrialsystolic event; controlling the pulse generator to deliver a ventricularpacing pulse upon expiration of the atrioventricular pacing interval;determining a metric from the motion signal correlated to a reliabilityof atrial systolic event detection; switching from the atrial trackingpacing mode to a non-atrial tracking pacing mode in response to themetric satisfying pacing mode switching criteria; and controlling thepulse generator to deliver ventricular pacing pulses in the non-atrialtracking pacing mode by controlling pulse generator to deliver aventricular pacing pulse at the sensor indicated pacing rate interval inresponse to the sensor indicated pacing rate interval being equal to orgreater than the minimum pacing interval.
 12. The pacemaker of claim 1,wherein the control circuit is configured to determine the sensorindicated pacing rate interval based on the motion signal.
 13. A methodperformed by an intracardiac ventricular pacemaker having a motionsensor configured to produce a motion signal, the method comprising:identifying a ventricular systolic event by a control circuit of thepacemaker; detecting by the control circuit a ventricular passivefilling event signal from the motion signal produced by the motionsensor within a ventricular chamber of the patient's heart; determininga time interval from the ventricular systolic event to the ventricularpassive filling event; establishing a minimum pacing interval based onthe time interval from the ventricular systolic event to the ventricularpassive filling event, the minimum pacing interval corresponding to amaximum ventricular pacing rate; determining a sensor indicated pacingrate interval; comparing the sensor indicated pacing rate interval tothe minimum pacing interval; determining that the sensor indicatedpacing rate interval is less than the minimum pacing interval; andcontrolling a pulse generator to deliver a non-atrial trackingventricular pacing pulse at the minimum pacing interval corresponding tothe maximum ventricular pacing rate in response to the sensor indicatedpacing rate interval being less than the minimum pacing interval. 14.The method of claim 13, further comprising: controlling the pulsegenerator to deliver ventricular pacing pulses in an atrial trackingpacing mode by: detecting an atrial systolic event from the motionsignal, setting an atrioventricular pacing interval in response todetecting the atrial systolic event, and controlling the pulse generatorto deliver a ventricular pacing pulse upon expiration of theatrioventricular pacing interval; and determining the time interval fromthe ventricular systolic event to the ventricular passive filling eventduring the atrial tracking pacing mode.
 15. The method of claim 14,further comprising: determining by the control circuit that pacing modeswitching criteria are satisfied; switching from controlling the pulsegenerator in the atrial tracking pacing mode to controlling the pulsegenerator in a non-atrial tracking pacing mode in response to the pacingmode switching criteria being satisfied; and determining the sensorindicated rate during the non-atrial tracking pacing mode.
 16. Themethod of claim 15, further comprising: setting an atrial refractoryperiod in response to identifying the ventricular systolic event;detecting the ventricular passive filling event from the motion signalduring the atrial refractory period; and detecting the atrial systolicevent outside the atrial refractory period.
 17. The method of claim 13,further comprising: detecting a resting stable heart rate; anddetermining the time interval from the ventricular systolic event to theventricular passive filling event during the resting stable heart rate.18. The method of claim 13, further comprising: determining the timeinterval from the ventricular systolic event to the ventricular passivefilling event for each of a plurality of heart rates; extrapolating acorrected time interval from the ventricular systolic event to theventricular passive filling event for a heart rate greater than theplurality of heart rates; and establishing the minimum pacing intervalbased on the corrected time interval.
 19. The method of claim 13,further comprising: determining a corrected time interval from theventricular systolic event to the ventricular passive filling eventbased on a function of the time interval and a target heart rateinterval; and establishing the minimum pacing interval based on thecorrected time interval.
 20. The method of claim 13, further comprisingidentifying the ventricular systolic event from the motion signal. 21.The method of claim 13, further comprising: sensing an R-wave from acardiac electrical signal received by a sensing circuit configured ofthe pacemaker, wherein identifying the ventricular systolic eventcomprises identifying one of a ventricular pacing pulse and the sensedR-wave.
 22. The method of claim 13, further comprising: determining anensemble averaged motion signal from the motion signal acquired over aplurality of cardiac cycles; and determining the time interval from theventricular systolic event to the ventricular passive filling event fromthe ensemble averaged motion signal.
 23. The method of claim 13, furthercomprising: controlling the pulse generator to deliver ventricularpacing pulses in an atrial tracking pacing mode by: determining anensemble averaged motion signal from the motion signal acquired over aplurality of cardiac cycles; determining the time interval from theventricular systolic event to the ventricular passive filling event fromthe ensemble averaged motion signal; setting an atrial refractory periodbased on the time interval; detecting an atrial systolic event from themotion signal outside the atrial refractory period; setting anatrioventricular pacing interval in response to detecting the atrialsystolic event; controlling the pulse generator to deliver a ventricularpacing pulse upon expiration of the atrioventricular pacing interval;determining a metric from the motion signal correlated to a reliabilityof atrial systolic event detection; switching from the atrial trackingpacing mode to a non-atrial tracking pacing mode in response to themetric satisfying pacing mode switching criteria; and controlling thepulse generator to deliver ventricular pacing pulses in the non-atrialtracking pacing mode by controlling pulse generator to deliver aventricular pacing pulse at the sensor indicated pacing rate interval inresponse to the sensor indicated pacing rate interval being equal to orgreater than the minimum pacing interval.
 24. The method of claim 13,wherein determining the sensor indicated pacing rate interval comprisesdetermining the sensor indicated pacing rate interval based on themotion signal.
 25. A non-transitory, computer-readable medium storing aset of instructions, which, when executed by a control circuit of anintracardiac ventricular pacemaker having a motion sensor configured toproduce a motion signal, cause the pacemaker to: identify a ventricularsystolic event; detect a ventricular passive filling event signal fromthe motion signal produced by the motion sensor within a ventricularchamber of a patient's heart; determine a time interval from theventricular systolic event to the ventricular passive filling event;establish a minimum pacing interval based on the time interval from theventricular systolic event to the ventricular passive filling event, theminimum pacing interval corresponding to a maximum ventricular pacingrate; determine a sensor indicated pacing rate interval; compare thesensor indicated pacing rate interval to the minimum pacing interval;determine that the sensor indicated pacing rate interval is less thanthe minimum pacing interval; and deliver a non-atrial trackingventricular pacing pulse at the minimum pacing interval corresponding tothe maximum ventricular pacing rate in response to the sensor indicatedpacing rate interval being less than the minimum pacing interval.